Level measurement forms a central part in many industrial applications. Level measurement data is for example used for industrial process control and/or automation. Various level measurement devices are presently available on the market.
There are level measurement devices on the market, applying time of flight radar techniques to determine the position of the surface of a product in a container. Corresponding level measurement devices comprise a conductive probe that extends into the container. The probe is connected to sensor electronics comprising means for generating and sending short electromagnetic pulses down the probe and means for reception of echoes of the pulses reflected at a surface of the product. The level of the product is determined based on a measurement of a time of flight needed for a pulse to travel down the probe and its echo to return. Such devices are for example the Guided Radar Transmitters Levelflex FMP 40, sold by the applicant, or devices as they are for example described in US-A6,690,320 or DE-A1 100 19 129.
In typical measurement arrangements, a level sensor comprising the conductive probe has a mounting element including a process connector, for mounting the mounting element on a mating ferrule on the container. The conductive probe is isolated from the process connector and the mating ferrule by an electric insulator inside the mounting element and/or the process connector, e.g. by an insulated feed through inside the process connector. A frequently used insulating material is for example polytetrafluorethylen (PTFE). The mating ferrule often comprises a tubular stud or hollow cylinder, enclosing an opening of the container, and a process connector, e.g. a flange, a threaded coupling or a tri-clamp connector, onto which the level sensor is mounted via its process connector. The conductive probe extends through the mating ferrule into the container. Typically a cavity exists between the conductive probe and the sections of the ferrule below its process connector.
The cavity is open to the container. Consequently, there is a possibility, that a product inside the container can enter the cavity. For example, if liquids and powders are mixed inside the container, the materials can be splashed into the cavity. Conductive products entering the cavity can form a conductive path between the conductive probe and the ferrule, which can seriously affect transmission of electromagnetic signals, in particular surface waves, up and down the probe or in the worst case make it impossible. Because of deposits in the vicinity of the probe and the ferrule, undesired reflections, signal attenuation and/or noise may occur. A short circuit in this region renders signal transmission impossible. Quite often, the ferrule and the process connector of the mounting element are connected via metal parts, which form an electrically conductive connection between the process connector and the ferrule. A short circuit between the probe and the ferrule would then short circuit the probe and the process connector of the mounting element.